The present invention relates to a high efficiency, linear DC power supply. More particularly, the present invention relates to an innovative linear DC power supply which takes a higher DC (or rectified ac) voltage from a source, reduces it to a lower DC voltage with low regulation and electrical noise for a load, includes a pulse-width-modulation (PWM) stage and a linear voltage regulation stage arranged in series, with the PWM stage and tile linear voltage regulation stage coupled with an energy storage means, and coordinated by a feedback circuit.
The present invention draws power from essentially any DC or rectified AC source, outputs DC power to essentially any type of load (though is optimized for electronic loads), offers high efficiency as compared with past art, attenuates the electrical noise inherent in a DC or rectified AC source equal to or better than past art, adds little or no electrical noise of its own, and can be built with fewer and less expensive parts. Because of this combination of traits, the present invention is useful in a wider range of applications and at lower cost, and thereby represents a new and useful invention.
The use of DC electronic appliances such as telephone answering machines and portable computers in mobile applications (recreational vehicles, autos, boats, etc) and in solar electric applications has risen dramatically in recent years. Although such appliances might be powered by dedicated batteries, power derived from a mobile or solar electric battery bank is often more convenient and economical to use.
A power supply is usually required for satisfactory and reliable operation of such electronic appliances in such environments. Battery bank voltage must usually be reduced to a lower level, delivered voltage held steady (regulated) regardless of variations in bank voltage or current draw by the appliance, and transient voltages which might be generated by a variety of sources and broadly referred to as "noise" attenuated.
For example, charging typically raises battery bank voltage by about a volt, heavy loads which share the bank can pull down voltage by an equal amount, and ignition coils and motors routinely generate potentially interfering and damaging transient voltages. Moreover, using a higher voltage boost to start a engine or connecting jumper cables improperly may cause immediate damage. Meanwhile, current draw by a typical electronic load such as a telephone answering machine may vary from 100 milliamperes when idle, to 1.2 amperes as the tape drive motor first activates.
Prior art for DC power supplies falls into several basic categories. Dropping resistors are perhaps the simplest and least expensive solution. For example, placing a, 12 ohm dropping resistor between a 24 volt battery bank and a 12 volt appliance drawing one ampere of current results in correct voltage delivery. However, as much power is dissipated in this dropping resistor as in the load, bank variations and noise are attenuated by only half, and delivered voltage changes as the appliance draws more or less current. A voltage divider, wherein the dropping resistor is referenced to both the positive and negative terminals of the bank and the load connected intermediate, somewhat stabilizes delivered voltage. However, improvement is at further expense to efficiency.
Although tapping individual cells of a battery bank to reduce voltage leads to imbalance, a Vanner power supply effectively splits a bank employing two identical batteries connected in series in half. Power is drawn first from one battery, and then from the other. While efficiency is high and delivered voltage as the appliance draws more or less current steady, only a single voltage can be obtained (half the series voltage), and bank voltage variations and noise are attenuated by no more than half.
Another solution is using a DC inverter to convert battery bank voltage to AC, and then using an AC-to-DC adaptor to convert the result back to DC at the correct voltage. Though credible, this solution is probably too expensive and bulky for most applications.
Buck regulators, which are becoming popular in prior art, provide DC voltage reduction at efficiencies in the low to mid 80 percent range, good regulation, and good attenuation of noise from external sources. Typically, a Buck regulator outputs high frequency PWM pulses (in the 50 kHz range) into what is essentially a dampened LC filter. Because the effectiveness of reactive elements increases directly with frequency, only relatively small values of L and C are needed. A DC output for the load is taken directly from this filter. Regulation is accomplished as the instantaneous voltage developed across this filter is fed back to the regulator, compared with a reference voltage, and PWM adjusted accordingly.
One disadvantage of Buck regulators is that because PWM is at radio frequency, switching losses within the regulator are invariably substantial. Another disadvantage is that the LC filter never completely smooths the PWM pulses, particularly as the load draws more current. For this reason, a Buck regulator is not useful with an electronic appliance having components operating near the PWM oscillation frequency (such as an am radio) or some harmonic multiple. A further disadvantage is that means for interference rejection such as shielding, use of short leads, and positioning of components may be necessary to minimize unwanted electric, magnetic, and electromagnetic coupling.
A linear voltage regulator is perhaps the best solution if relatively little load current is required. For example, a widely available and inexpensive three terminal linear voltage regulator reduces supply voltage to a preset or adjustable lower level, regulates for both bank and load variations thereby holding delivered voltage steady, and attenuates noise. Viewed with an oscilloscope, a straight line (linear voltage) is seen across the load.
However, like dropping resistors, linear voltage regulators dissipate energy in proportion to voltage dropped. For example, a 4 watt (internal dissipation) linear voltage regulator drawing power from a 24 volt battery bank and supplying power to a 6 volt load has an efficiency of less than 25 percent, and can continuously output no more than about 0.22 amperes of current. Although units capable of supplying more current and dissipating more heat are available, use is infrequent because of higher cost, the need for expensive heat sinking and perhaps a cooling fan, and battery bank drain.
A linear voltage regulator needs to receive an operating voltage which is high enough to power the load, provide for internal operation, and stay below the noise level which may be present in the supply. Voltage in excess of this amount serves no useful purpose, and is simply dissipated as heat. For example, the optimal operating voltage for a particular regulator powering a 6 volt load might be 7.5 volts, of which 6 volts goes to the load, 0.5 volts to internal operation, and 1 volt to staying below noise levels.